A lithium secondary battery has advantages such as high energy density, low self-discharge rate and low-weight, and thus receives a great deal of attention as a high performance energy source for portable electronic instruments requiring slimness and low-weight such as a notebook computer, a camcorder, a mobile phone, and the like. This lithium secondary battery generally uses mixed oxides of lithium metal as its cathode active material, and carbon material or metallic lithium as its anode material, respectively. The lithium battery is typically designed with a high pressure, and thus an organic solvent capable of withstanding a high voltage, that is, a nonaqueous electrolyte, is employed as the electrolyte. In this connection, the nonaqueous electrolyte in which lithium salts are dissolved in an organic solvent is primarily used.
The electrolyte for the lithium battery is required to be stable against a lithium anode, but it is said that there is no solvent showing a thermodynamic stability to lithium. In practice, there has been considered that the electrolyte is decomposed relative to the anode upon initial charging and resulting products form an ionic conductivity protection film, i.e., SEI (Solid Electrode Interface) on a surface of lithium and then inhibits a reaction between the electrode and electrolyte thereby stabilizing the battery.
However, in case of such a nonaqueous electrolyte based secondary battery, if a power circuit or a charger of electronic instruments is out of order or overcharged, aberrant heat generation occurs in the battery and further, in extreme case, damage or catching fire of the battery may occur. As such, it is an important problem to effectively inhibit such heat generation and secure stability of the battery so as to prevent thermal runaway, upon abnormal operation such as overcharging the battery.
As a measure to prevent rupture and catching fire of the battery upon overcharging thereof, there has been to control primarily a charging voltage of the battery by a charger. However, at the present time, since use of a protection circuit, a protection device, and the like, significantly limits miniaturization and reduced production costs of a battery pack, there is an urgent need for a method capable of securing stability of the battery without the protection circuit or protection device. In addition, there have been proposed cutting off electric current by gas production when overcurrent is generated in the battery, or cutting off overcharge current by fusion of a separator, but a satisfactory protection mechanism to prevent overcharge has yet to be realized.
In order to overcome these problems, Japanese Patent Publication Nos. Hei 7-302614, 9-50822 and 9-106835, and Japanese Patent No. 2939469 have proposed methods to ensure stability against overcharging of the battery by adding a small amount of an aromatic compound to the electrolyte of the lithium secondary battery.
More specifically, Japanese Patent Publication Nos. Hei 7-302614 and 9-50822 propose the use of a lower molecular weight organic compound with a molecular weight of less than 500, such as anisole, having a reversible redox potential at above a cathode potential upon full charging of the secondary battery, and a π electron orbit, in the electrolyte as an additive. Such a lower molecular weight organic compound serves as a redox shuttle and consumes overcharge current between the cathode and anode to establish a protection mechanism.